As a relatively high-power secondary battery, a battery module is hitherto known in which a plurality of battery cells are stacked and assembled as a unit and are connected to one another in series or in parallel.
It is known in the art that a gas is generated inside a battery cell in accordance with the deterioration with time, and that the internal pressure in the battery cell increases, expanding the case of that battery cell. In a battery module wherein a plurality of battery cells are stacked one upon another, the case of a battery cell may expand, and the battery module may change in outer shape. It is also known that the battery module whose outer shape has changed has poor performance. In order to suppress the expansion of each battery cell, the stacked battery cells are constrained using a frame.
However, if the frame is too rigid, the battery module may not be assembled efficiently. Conversely, if the frame is not sufficiently rigid, expansion of the battery cells of the battery module cannot be suppressed.
Under the circumstances, there is a demand for a battery module in which the rigidity of the frame for constraining the battery cells increases in accordance with the expansion of the deteriorated battery cells and which can be assembled easily.